1 Chronicles 23:5-7

Another 4,000 will work as gatekeepers, and 4,000 will praise the LORD with the musical instruments I have made.” Then David divided the Levites into divisions named after the clans descended from the three sons of Levi—Gershon, Kohath, and Merari. The Gershonite family units were defined by their lines of descent from Libni and Shimei, the sons of Gershon.